“What was I looking for again?”

A coworker comes to your office to ask a question. You were supposed to search for some information for her on your computer, but you got sidetracked. You forgot what you were looking for. You say this to yourself.

There's an old rule that some English teachers might try to enforce, which says that you shouldn't end a sentence with a preposition like "for". According to this rule, you're supposed to say or write:

For what are you looking?

But this is really strange and old-fashioned. You can use it in extremely formal writing, or maybe as a joke.